Generally, a part of films, such as cellulose acetate, polycarbonate, and cellophane are produced by solvent casting. In the solvent casting process, dope being a polymer solution is cast from a casting die onto a casting support of belt type or drum type, stripped off after being dehumidified to some extent on the casting support, dried while being carried by a tenter machine with being held at both side edges, and further dried while being conveyed over rollers (JP 62-46625A, JP 62-46626A, etc.).
In order to improve productivity in such a solvent casting process, it is necessary to rapidly strip off the dope cast on the casting support. As a result, various technics for stripping off the dope quickly have been developed, such as, a successive multiplayer casting (JP 52-50078A, JP 53-134869A and JP 61-104813A), a casting of a gelation dope on a cooling drum JP 5 17844B and JP 62-64514A), a method of co-casting dopes wherein a gelation dope is cast on the side of the casting support (JP 61-94724A), etc.
In all the foregoing conventional methods, however, it is necessary for the cast film to have self-supporting ability at the time of being stripped off from the casting support. Therefore, particularly great drying rate is required between the casting and the point of being stripped off. Further, it is desirable that the drying of the stripped off film is conveyed by a non-contact system, like a tenter machine. However, it was found that, according to drying conditions, problems occur, such as foaming and degradation of film properties, such as, mechanical strength, transparency, retardation, flatness, etc.
On the other hand, in the case that the cast film containing volatile components in a high concentration is stripped off from a casting support by a tenter machine of a clip type, the film breaks at the portion caught by the clip, and it is difficult to continue earring the film by the tenter. In particular, such a phenomena tends to occur upon loading tension to the film being carried by the tenter in the width direction, or stretching the film in the width direction. Therefore, the high-speed casting and improvement in the manufacturing speed of the film are difficult. In addition, since the film stripped off from the casting support contains volatile components in quality, the film is occasionally foamed at the part caught by a clip tenter heated to a high temperature by drying wind. Then, the foams generated in the film edges are possible to extend to product portion, which become a problem in quality. Moreover, break of the film also occurs occasionally caused by rupture of foamed portions.
In a tenter machine of pin type, many pins penetrate through both side edge portions of the film. When the thickness of this penetration area of the film is thin, the film edge tears by the tension caused by contraction on drying, and it is also possible that the film breaks while releasing the film from the tenter machine. On the other hand, when the thickness of this penetration portion of the film is too thick, the pins are contaminated by the adhesion of the dope not sufficiently dried in the film, which causes a conveyance trouble.